This invention relates to a method of controlling the temperature of a flue gas stream entering a Selective Catalytic Reduction chamber in a steam generating power plant that utilizes Selective Catalytic Reduction (SCR) to lower NO.sub.x emissions and more specifically to the utilization of superheated steam to control the temperature of the flue gas prior to the flue gas entering a SCR chamber such that there is a reduction in lost energy to the thermodynamic steam cycle caused by spray desuperheating.
In recent years oxides of nitrogen, also known as NO.sub.x, have been implicated as one of the elements contributing to the generation of acid rain and smog. Now, due to very strict state and federal environmental regulations demanding that NO.sub.x emissions be maintained at acceptable levels, the reduction of NO.sub.x both during and after the combustion process is of critical importance and a major concern in the design and operation of modern power plants. Oxides of nitrogen are a byproduct of the combustion of hydrocarbon fuels, such as pulverized coal, gas or oil and are found in two main forms. If the nitrogen originates from the air in which the combustion process occurs, the NO.sub.x is referred to as "thermal NO.sub.x." Thermal NO.sub.x forms when very stable molecular nitrogen, N.sub.2, is subjected to temperatures above about 2800.degree. F. causing it to break down into elemental nitrogen, N, which can then combine with elemental or molecular oxygen to form NO or NO.sub.2.
If the nitrogen originates as organically bound nitrogen within the fuel, the NO.sub.x is referred to as "fuel NO.sub.x." The nitrogen content of coal, for instance, is comparatively small and, although only a fraction is ultimately converted to NO.sub.x, it is the primary source of the total NO.sub.x emissions from a coal-fired steam generating power plant.
One post-combustion process for the lowering of NO.sub.x emissions is that of Selective Catalytic Reduction (SCR). Selective Catalytic Reduction systems use a catalyst and a reactant such as ammonia gas, NH.sub.3, to dissociate NO.sub.x to molecular nitrogen, N.sub.2, and water vapor. The catalytic process using ammonia as a reactant is governed by the following primary chemical reactions: ##EQU1## Since NO.sub.x is approximately 95% NO in the flue gas stream, Equation 1 dominates the process.
Urea is a promising SCR reactant that is coming into use in lieu of ammonia. The catalytic process using urea as the reactant is governed by the following chemical equation: ##EQU2##
A typical utility steam generating power plant utilizing Selective Catalytic Reduction as a NO.sub.x reduction technique comprises a furnace volume in fluid communication with a backpass volume. Combustion of hydrocarbon fuels occurs within the furnace volume creating hot flue gases that rise within the furnace volume giving up a portion of their energy to the working fluid of a thermodynamic steam cycle. The flue gases are then directed to and through the backpass volume wherein they give up additional energy to the working fluid. Upon exiting the backpass volume the flue gases are directed via a gas duct through a Selective Catalytic Reduction chamber and thence to an air preheater and flue gas cleaning systems thence to the atmosphere via a stack.
In a typical SCR system, at some point in the gas duct after the flue gas stream exits the backpass volume and upstream of the SCR chamber, a reactant, possibly ammonia, in a gaseous form, or a urea/water solution is introduced into, and encouraged to mix with, the flue gas stream. The reactant/flue gas mixture then enters the SCR chamber wherein the catalytic reductions, shown in Equations 1, 2 or 3 take place between the reactant/flue gas mixture and the catalytic material. The introduction of the ammonia or urea into the flue gas stream is typically achieved by the use of injector nozzles located at either the periphery of the gas duct, or immersed within the flue gas stream.
The design of a SCR system is dictated by such considerations as the SO.sub.3 concentration, as well as the concentration of NO.sub.x entering and leaving the SCR chamber, the flue gas temperature, the ammonia/NO.sub.x stoichiometric ratio, the flue gas volumetric flow rate, the flue gas velocity, the flue gas oxygen and moisture content and the available surface area of the catalytic material. The SCR reaction chamber typically includes multiple layers of solid catalytic material lying within the path of the flue gas stream. The most common types of catalytic material in use and the approximate temperature ranges of the flue gas over which they are effective as catalysts are: Titanium Dioxide (270-400.degree. C.), Zeolite (300-430.degree. C.), Iron Oxide (380-430.degree. C.) and activated coal/coke (100-150.degree. C.).
Maintaining a minimum flue gas temperature helps prevent the formation of ammonium bisulfate and ammonium sulfate salts on the surface of the catalytic material due to any unreacted ammonia present in the flue gas. Such a formation reduces the effectiveness of the Selective Catalytic Reduction process. The type and amount of catalytic material for which a SCR system need be designed depends upon the flue gas volume, flue gas temperature, total NO.sub.x present in the flue gas, NO.sub.x reduction requirements, permissible ammonia slip, catalyst life requirements, amount of SO.sub.x present in the flue gas stream, ash loading in the flue gas and the uniformity of the temperature, velocity and concentration of the reactant in the flue gas stream as the mixture enters the SCR chamber.
The use of Selective Catalytic Reduction in lowering NO.sub.x emissions is not new. In particular U.S. Pat. No. 5,151,256, entitled "Coal Combustion Apparatus Provided With A Denitration" and which issued on Sep. 29, 1992, relates to a coal combustion apparatus devised so that a denitration catalyst can be hardly poisoned by volatile metal compounds contained in exhaust gases in a denitration means for catalytic reduction with ammonia, and a method for eliminating said volatile metal compounds from said exhaust gases.
Furthermore, it is seen that U.S. Pat. No. 5,233,934, entitled "Control Of NO.sub.x Reduction Flue Gas Flows" and which issued on Aug. 10, 1993, discloses an invention that relates to the reduction of pollutants produced by boilers, and, more particularly, to the control of the process for reducing NO.sub.x pollutants in the flue gas flows.
Further still, U.S. Pat. No. 5,237,939, entitled "Method And Apparatus For Reducing NO.sub.x Emissions" and which issued on Aug. 24, 1993, discloses an apparatus that reacts NO.sub.x in a flue gas stream with a nitrogenous compound such as a source of ammonia in the presence of a catalyst to reduce the NO.sub.x level of the flue gas.
Still further, U.S. Pat. No. 5,296,206, entitled "Using Flue Gas Energy To Vaporize Aqueous Reducing Agent For Reduction Of NO.sub.x In Flue Gas" and which issued on Mar. 22, 1994, discloses a method of vaporizing aqueous reducing agent for reducing NO.sub.x in flue gas originating in a combustion in which the NO.sub.x is generated.
During the operation of a steam generating power plant at low boiler loads, the minimum required temperature of the flue gas necessary for effective Selective Catalytic Reduction may not be attainable. To help ensure that the temperature of the flue gas stream is within the aforesaid temperature ranges while at low boiler loads, it is typical that a bypass duct is utilized to reheat the flue gas stream. The bypass duct typically passes from the backpass volume to the gas duct such that still relatively hot flue gases are diverted from the backpass volume to a point in the gas duct upstream of the location of the injection of the reactant into the SCR chamber.
The prior art has recognized the need to reheat flue gases that are generated from the combustion of fossil fuels as demonstrated prior art efforts directed toward the reduction of NO.sub.x and SO.sub.x emissions generated from the combustion of fossil fuels. In particular, with respect to the use of a bypass duct, U.S. Pat. No. 3,320,906, entitled "Fuel Burning Process And Apparatus" and which issued on May 23, 1967, discloses a technique for removing the sulfur compounds and particulate matter from the products of combustion or flue gases of steam generating and similar equipment and reheating the flue gases prior to discharge to the atmosphere. Further in the prior art U.S. Pat. No. 4,160,009 provides a boiler apparatus having a furnace and a plurality of heat exchangers disposed in a combustion gas channel between the furnace and boiler apparatus exits, the improvement comprising a denitrator having a catalyst disposed in the combustion gas channel downstream of at least one of the heat exchangers, a bypass duct for the combustion gas channel connecting a first region thereof in which the denitrator is disposed with a second region upstream of the first region, control valve means disposed in the duct, and a temperature detector disposed in the first region and connected to the control valve means so as to control the opening and the closing of the valve means in response to the temperature detected in the first region by the detector.
Further in the prior art, U.S. Pat. No. 4,300,920, entitled "Stack Gas Reheater System" and which issued on Nov. 17, 1981, discloses a stack gas reheater system of the type where saturated flue gas is reheated in a reheat heat exchanger to prevent condensation in the stack and in the exhaust fan which conveys the flue gas to the stack. U.S. Pat. No. 4,300,920 further relates to a reheater system whereby further additional heat is added to the flue gas stream upstream of the reheat heat exchanger to prevent condensation in the reheat heat exchanger whose primary function is indicated above.
Still further, U.S. Pat. No. 4,310,498, entitled "Temperature Control For Dry SO.sub.2 Scrubbing System" and which issued on Jan. 12, 1982, discloses an apparatus and method for removing sulfur oxides from a flue gas produced during the combustion of a sulfur bearing coal fossil fuel. Also, U.S. Pat. No. 4,310,498 relates to an apparatus and method for controlling the temperature of the flue gas entering a spray dryer absorption chamber in order to be sprayed into the flue gas thereby allowing the treatment of flue gas containing high levels of sulfur oxide and insuring higher sulfur removal efficiencies.
U.S. Pat. No. 4,705,101, entitled "Flue Gas Reheat Apparatus" and which issued on Nov. 10, 1987, discloses an invention that relates to flue gas scrubbing and reheating apparatus, and more particularly, to improved method and apparatus which provides important economic advantages.
U.S. Pat. No. 5,555,849, entitled "Gas Temperature Control System For Catalytic Reduction Of Nitrogen Oxide Emissions" and which issued on Sep. 16, 1996 to the same assignee as the present invention, relates to the catalytic reduction of nitrogen oxide emissions from fossil-fuel power plants and more particularly to the control of the flue gas temperature entering the catalytic reactor during low load operations.
However, the use of a bypass duct at low boiler loads, such as would be used in a fuel-fired steam generating power plant to reheat flue gases, offers fewer advantages than under other boiler load conditions. Diverting relatively hot flue gases to be mixed with relatively cool flue gases creates temperature differentials that give rise to thermal stresses in the backpass volume, the bypass duct and the gas duct leading to the SCR chamber. These thermal stresses are usually relieved by way of expansion joints. In addition, control dampers within the bypass duct are required in order to regulate the flow of the flue gas through the bypass duct. Furthermore, a slide gate is typically used to seal off the bypass duct when it is not in use. However, due to warpage caused by thermal expansion, and fouling due to the accumulation of fly ash entrained within the flue gas, the slide gate may not seal properly. This causes flue gas leakage when the slide gate is closed and is a source of the need to regularly replace seals. Still further, the use of a bypass duct to control the temperature of the flue gas entering the SCR chamber creates temperature gradients across the flue gas stream as it enters the SCR chamber. A more uniform temperature profile across the flue gas stream is required to allow for a more efficient use of the catalytic material in the Selective Catalytic Reduction process.
Current methods of steam generation in the thermodynamic steam cycle of a fuel-fired steam generating power plant may typically direct superheated steam from a primary superheater to a desuperheater and thereafter to a secondary superheater before being directed to a high pressure turbine for expansion therein. However, at various boiler loads, the temperature of the superheated steam directed to the high pressure turbine may be too high. In order to lower this temperature to an optimum value, water spray is added to the superheated steam, as needed, by a desuperheater. Such spray desuperheating usually occurs before the secondary superheater and represents a loss in useful energy to the thermodynamic steam cycle.